US 2004/0177102 corresponding to JP 4074823 discloses a signal processing unit including a time analog-to-digital converter (hereinafter called the “TAD”) and a correction circuit. In the signal processing circuit, multiple analog voltage signals are selectively inputted by an analog multiplexer to the TAD. The TAD sequentially converts the inputted analog voltage signals to digital outputs. The TAD has nonlinear input and output characteristics that can cause a conversion error. The correction circuit applies a linear correction to the output of the TAD by using at least three reference voltages. Specifically, the correction circuit uses the minimum voltage Vmin, the maximum voltage Vmax, and the center voltage Vc.
By the way, a thermal air flow meter (i.e., hot-wire flow meter) has been widely used to measure intake air flow to an engine of a vehicle. In a conventional thermal air flow meter, heat of a heat generating element is dissipated by the air flow, and the air flow is measured by using the principle that there is a correlation between the amount of dissipated heat and the amount of the air flow. However, since the amount of the air flow is measured as an absolute value, the direction of the air flow cannot be detected. For example, in a four-cylinder engine, intake air pulsation increases at low speed and high load operations, and a reversal of the direction of air flow may occur. In the conventional flow meter, since the air flow is measured without consideration of the direction of the air flow, the amount of intake air flow to a firing chamber of the engine cannot be accurately measured.
US 2009/0299657 corresponding to JP-A-2009-288153 discloses a system including a thermal air flow meter for measuring air flow by detecting the direction of the air flow. The system includes a TAD and a signal processing unit. The TAD converts an output of the flow meter into a digital signal. The signal processing unit applies correction processing to the digital signal and outputs the corrected digital signal to external device such as an electronic control unit (ECU).
The flow meter includes a heat generating element and a temperature detection element. The heat generating element is located in the center of a thin-film portion of a silicon substrate. The temperature detection element is located on upstream and downstream sides of the heat generating element with respect to the flow of intake air. A temperature control circuit controls the temperature of the heat generating element so that the temperature of the heat generating element can be greater than an intake air temperature by a predetermined value. The air flow is measured by calculating a difference between a temperature detected by the temperature detection element located on the upstream side and a temperature detected by the temperature detection element located on the downstream side. The calculated difference changes sign, when the direction of the air flow is reversed. Therefore, the direction of the air flow can be detected based on the sign of the calculated difference. It is noted that the air flow is measured as mass flow rate [g/sec].
Specifically, the heat generating element of the flow meter is a temperature-sensitive resistor having a temperature dependence and incorporated in a bridge circuit. The heating current flowing through the heat generating element is controlled by the temperature control circuit so that the temperature of the heat generating element can be kept greater than the ambient temperature (i.e., intake air temperature) by a predetermined value.
However, output characteristics and temperature characteristics vary from flow meter to flow meter. The signal processing unit corrects the individual variation of the flow meter so that the corrected signal can be outputted to the ECU.
The present inventors considered that the signal processing unit disclosed in US 2004/0177102 is applied to the system disclosed in US 2009/0299657 to digitally correct the individual variation. However, the present inventors found the following disadvantages.
In the signal processing unit disclosed in JP-4074823, five analog voltage signals are selectively inputted to the TAD, and the TAD sequentially converts the analog voltage signals into digital data (hereinafter called the “TAD output data”). The TAD output data is inputted to the correction circuit, and the correction circuit applies correction processing, such as linear correction and temperature dependence correction, to the TAD output data. The corrected TAD output data is converted into a frequency signal and inputted to the ECU.
The five analog voltage signals includes three reference voltage signals Vref1, Vref2, and Vref3, an intake air temperature voltage signal Vt, and an air flow voltage signal Vq. The intake air temperature voltage signal Vt is an output signal of an intake air temperature sensor, and the air flow voltage signal Vq is an output signal of the flow meter.
FIG. 10 is a flowchart of a program, created by the inventors, for applying the signal processing unit disclosed in US 2004/0177102 to the system disclosed in US 2009/0299657. The program is summarized below.
When the TAD starts an A/D conversion process, switches of an analog multiplexer are controlled so that the analog multiplexer can switch to the third reference voltage signal Vref3. Thus, the third reference voltage signal Vref3 is inputted to and sampled by the TAD. Then, the third reference voltage signal Vref3 is converted into digital data, and the digital data corresponding to the third reference voltage signal Vref3 is stored in a register at step S81.
Then, the second reference voltage signal Vref2, the first reference voltage signal Vref1, the intake air temperature voltage signal Vt, and the air flow voltage signal Vq are processed in the same manner as the third reference voltage signal Vref3 at steps S82, S83, S84, and S85, respectively.
Thus, the five analog voltage signals are inputted to the TAD with a constant period of T1. In other words, five steps S81-S85 are repeated with the constant period of T1. The period of T1 is a time between the same steps.
Further, steps S91-S93 are performed in parallel with steps S81-S85.
At step S91, correction processing including linear correction disclosed in US 2004/0177102 and temperature dependence correction disclosed in US 2009/0299657 is applied to the TAD output data.
At step S92, the corrected TAD output data is converted into the frequency signal.
At step S93, the frequency signal is outputted to the ECU.
According to the program shown in FIG. 10, the five analog voltage signals are sequentially sampled and converted with the constant period of T1. That is, each of the five analog signals including the air flow voltage signal Vq is sampled and converted with the constant period of T1.
Accordingly, the air flow voltage signal Vq as an output signal of the flow meter is sampled and converted with the constant period of T1. Therefore, as shown in FIG. 9B, it may be difficult to follow the change in the air flow under high frequency pulsation caused when an engine rotates at high speed.